Mobile phones enable their users to conduct conversations in different acoustic environments, some of which are relatively quiet, while others are quite noisy. To improve intelligibility of the far-end user's speech to the near-end user who is in a hostile acoustic environment, that is an environment in which the ambient acoustic noise or unwanted sound surrounding the mobile phone (also referred to here as background sound or background noise) is particularly high, such as on a busy street or near an airport or train station, an audio signal processing technique known as active noise cancellation (ANC) can be implemented in the mobile phone. A goal of ANC is to cancel or at least reduce the background sound that is heard by the near end user, for example, through his ear, which is pressed against an earpiece of a handset or is carrying an earphone, by producing an anti-noise signal that is designed to cancel (acoustically) the background sound. Typically, the anti-noise signal is driven through an earpiece speaker that is being used to produce the desired audio. The ANC circuitry uses a microphone referred to as the “error microphone” that is placed inside a cavity that is formed between the user's ear and the inside of an earpiece shell. The error microphone picks up the background sound that has leaked into the cavity, in addition to the desired sound being emitted from the earpiece speaker. In addition, a reference microphone is typically placed on an exterior of the earpiece shell, in order to directly detect the background sound. An adaptive digital filter W is then used to estimate the unknown acoustic response between the reference microphone and the error microphone, so that the output of the adaptive filter W generates an anti-noise signal that is intended to cancel the background sound being heard by the user (and as picked up by the error microphone). An adaptive digital filter controller uses as input the signal from the reference microphone, as well as a representation of the acoustically combined anti-noise and background sound picked up by the error microphone, in order to adapt the filter W over time (e.g., during a phone call or other audio playback session) so that the “error” between the anti-noise and the background sound, as picked up by the error microphone, is reduced as much as possible.
Audio signal processing integrated circuits that can be used to implement the adaptive filter W and the adaptive filter controller have been developed. In such systems, the adaptive filter W has been implemented as a finite impulse response (FIR) digital filter having 128 taps, and an effective sampling rate of about 48 kHz (for sampling the output of the reference microphone).